The present invention relates to a device for measuring the refractive error of the human eye.
The refractive error of the human eye is represented by the refractive power of a spectacle lens used to correct abnormal refraction of the eye, and more specifically by the spherical power, the cylinder power and the cylinder axis errors thereof.
A variety of eye refractive error measuring devices have been proposed in the art. One such device is disclosed in the specification of U.S. Pat. No. 3,888,569 or Japanese Laid-Open Patent Application No. 52893/1979. In a conventional device, the image of a chart is projected onto an eye to be examined by means of an optical system and the state of the optical system at which the image of the chart formed on the retina of the eye becomes most sharp and clear is detected to thereby measure the refractive error of the eye. In the case of an astigmatic eye, the refractive error differs depending on the measuring longitude direction. Therefore, in the conventional device, refractive powers are measured at least in three longitudinal directions and the refractive errors (spherical power, cylinder power and cylinder axis errors) are obtained through calculation from the measured data. In order to carry out the measurement in a plurality of (three or more) longitudinal directions, according to the aforementioned U.S. Pat. No. 3,888,569, a chart is turned to be in various measuring longitudinal directions and the corresponding refractive powers in the various measuring longitudinal directions are measured in a time division manner. Therefore, the device according to U.S. Pat. No. 3,888,569 is disadvantageous in that it takes a relatively long time to achieve all the measurements.
In order to eliminate this difficulty, a technique has been proposed, as discussed in the above-noted Japanese Laid-Open Patent Application No. 52893/1979 whereby various charts are provided for measuring in the different longitudinal directions so that the measurements in all the longitudinal directions are achieved simultaneously. A difficult problem, however, remains to be solved, in that, in the realization of a practical refractive error measuring device, since the intensity of light interfered with by the retina is extremely low and the measurement is interrupted by light reflected by the cornea and the optical system, it is difficult to obtain a sufficiently high S/N ratio (ratio of signal to noise). As described in U.S. Pat. No. 3,888,569, this problem has been solved by scanning the chart itself thereby to separate signal light from noise light according to their frequencies. However, it is difficult to apply the method proposed by the U.S. Patent to the technique of the Japanese Laid-Open Patent Application No. 52893/1979. Thus, the drawback that a sufficiently high S/N ratio cannot be obtained still remains.
Accordingly, an object of the invention is to provide an eye refractive error measuring device in which refractive powers can be simultaneously measured in a plurality of longitudinal directions with a sufficiently high S/N ratio thereby to achieve the desired measurements in a short time and with a high accuracy.